1. Field of the Invention
The present invention relates to a system for controlling a warp feed in a loom in response to changes of weaving conditions.
2. Description of the Related Art
In the prior art, a weft density of a fabric is adjusted by cooperatively controlling take-up and let-off rates of a warp on a loom in accordance with a target weft density, as disclosed in Japanese Unexamined Patent Publication No. 62-263347. In the above document, the take-up rate and the let-off rate are taken as functions solely of the target weft density, and other weaving conditions are not taken into account. This is because it has been considered in the prior art that the weft density is merely an inverse proportional factor to the take-up or let-off rate of warp feed.
According to experiments conducted by the present inventors, it has been proved that the actual weft density in a fabric portion immediately after a change of the weaving condition has occurred is different from the target weft density. Namely, even if the target weft density is changed in a stepwise manner, the actual density varies gradually, whereby a considerably large area is formed in which the weft density is different from the target value.
This is because a position of a cloth fell, i.e., a boundary line between a warp region in which a weft has not been inserted and a fabric now being formed on a loom, moves by a certain distance until a steady state is reestablished corresponding to the change of the target weft density. Such a displacement of cloth fell position may be caused not only by the change of the target weft density but also by changes of other weaving conditions, such as the kind of weft or warp yarns or a warp tension.
Namely, as shown in FIG. 2(a), when a change of the weaving condition occurs, the cloth fell C is displaced from an original position L to a position L' or L" shown in chain lines, at which a new steady state of the cloth fell C is reestablished in accordance with a tension balance between warp and fabric in the vicinity of the cloth fell.
The above phenomenon will be explained in more detail with reference to FIG. 2(b). Assuming a displacement of the cloth fell position C per weft pick is .DELTA.L, then this value can be equivalent to a displacement rate .DELTA.V.sub.c of the cloth fell position, because the weft is picked at a constant period. The displacement rate .DELTA.V.sub.c may have either a positive value or a negative value, in accordance with the direction in which the cloth fell position C is moved. Accordingly, as shown in FIG. 2(b), a warp feed rate V is influenced by this .DELTA.V.sub.c as if the apparent warp feed rate V' becomes larger or smaller corresponding to the direction in which the cloth fell position C is moved. Note, the .DELTA.V.sub.c converges to zero as the steady state is reestablished under the new weaving conditions, but the disturbance of the weft density continues while the displacement of the cloth fell position continues. Even if it is desired to change the weft density in a stepwise manner, as illustrated in FIG. 2(c), the actual weft density gradually varies in accordance with the displacement of cloth fell position over a fabric length corresponding to about ten through twenty picks until the new cloth fell position is established.
Accordingly, in the prior art, a transition of the weft density cannot be avoided when the weaving condition has changed.